For example, a vehicle engine generally uses an electromotive force output type gas sensor which takes the exhaust gas discharged from the engine as an object of detection and detects the oxygen concentration. The gas sensor has an electrogenic cell which outputs an electromotive force signal which differs depending on whether the exhaust gas air-fuel ratio is rich or lean. Specifically, when the air-fuel ratio is rich, the gas sensor outputs an electromotive force signal of about 0.9 V and when the air-fuel ratio is lean, the gas sensor outputs an electromotive force signal of about 0 V.
As for this kind of gas sensor, attention has been drawn to the fact that when the air-fuel ratio of the exhaust gas changes to rich or lean, the sensor output changes with a delay from the actual change of the air-fuel ratio. Various techniques have been described to improve this output characteristic.
For example, in the gas sensor control device in Patent Literature 1, a constant current circuit is connected to at least one of a pair of sensor electrodes. When it is determined that a change request to change the output characteristic of the gas sensor has been generated, the direction of constant current is determined according to the change request and the constant current circuit is controlled so that the constant current flows in the determined direction. Thus, the output characteristic of the gas sensor is appropriately controlled by supplying the constant current.
In a gas sensor, the resistance value of the sensor element changes depending on the temperature of the sensor element. Specifically, when the engine is started in the cold or when the exhaust gas temperature decreases with fuel cut to the engine, the element resistance increases with the decrease in the temperature of the sensor element. In this case, as the element resistance increases, the voltage applied to the sensor element increases even under the condition that a constant current flows. When the applied voltage becomes excessive, a disadvantage (bad influence) such as deterioration of the solid electrolyte body of the sensor element may occur. Therefore, in the configuration in which a constant current is supplied to the sensor element, there is room for improvement from the viewpoint of protection of the sensor element. The sensor element corresponds to an electrogenic cell. The resistance value of the sensor element is also called the element resistance.